In the third generation partnership project long term evolution (3GPP LTE), as a technique to improve the communication quality of user equipment (UE) positioned at a cell edge, the “coordinated communication” by multiple cells has been considered. The coordinated communication is a technique of communicating with user equipment with multiple cells coordinating with each other. The coordinated communication in the 3GPP LTE is called “coordinated multi-point (CoMP)” communication (Emmanouil Pateromichelakis, Mehrdad Shariat, Atta ul Quddus, and Rahim Tafazolli, “On the Evolution of Multi-Cell Scheduling in 3GPP LTE/LTE-A”, IEEE Commun. Surv. & Tut., pp. 1-17, in press).
As downlink coordinated communication, for example, there is a “joint transmission (JT)” in which the same data is transmitted to a single user equipment from multiple transmission points each of which forms multiple cells. The transmission point is, for example, a base station, a remote radio head (RRH), or the like. Because gain improvement by the space diversity effect is achieved by performing this downlink coordinated communication, the reception quality is improved at user equipment positioned at a cell edge.
If all of cells around the user equipment are handled as candidate cells for the coordinated communication (hereinafter, “coordination candidate cells” in some cases), the processing amount at the base station, and the amount of signals communicated between the base station and the user equipment increase when determining a cell that actually performs the coordinated communication (hereinafter, “coordination cell” in some cases) from among the coordination candidate cells.
Specifically, for example, the base station allocates communication resources (that is, scheduling) to the user equipment based on a scheduling metric by 1 millisecond (ms) sub-frame unit, to determine a coordination cell per sub-frame. At this time, if all of the cells around the user equipment are determined as the coordination cells, the number of patterns of combinations of the coordination candidate cells is to be enormous, and the amount of calculation in the scheduling metric is to be enormous.
Moreover, when determining a coordination cell from among the coordination candidate cells, a reference signal (RS) for reception quality measurement by the user equipment is transmitted at all of the coordination candidate cells. The user equipment informs about the reception quality to all of the coordination candidate cells. Therefore, if all of the cells around the user equipment are determined as the coordination candidate cells, the overhead of the reference signal increases in the downlink, and the signaling overhead in informing the reception quality increases in the uplink.
Accordingly, a small number of cells are selected as the coordination candidate cells in advance. As a method of selecting a coordination candidate cell, there is a method that uses a reception power (reference signal received power (RSRP)) of a reference signal that is transmitted to user equipment at each cell. For example, among cells other than a serving cell, a cell having a difference in RSRP from RSRP of the serving cell that is equal to or smaller than a threshold is selected as a coordination candidate cell of the serving cell. The “serving cell” is, for user equipment, a cell in which the user equipment is positioned, that is, a cell in which the user equipment is currently registered.
Furthermore, selection of a coordination candidate cell is also called CoMP set setting, coordination area setting, or clustering.
Examples of related-art are described in Emmanouil Pateromichelakis, Mehrdad Shariat, Atta ul Quddus, and Rahim Tafazolli, “On the Evolution of Multi-Cell Scheduling in 3GPP LTE/LTE-A,” IEEE Commun. Surv. & Tut., pp. 1-17, in press.
As described above, when a coordination candidate cell is selected based on a difference in RSRP and a threshold, a coordination candidate cell to be selected is dependent on the threshold, and therefore, it is important to set an optimal threshold. However, because an optimal threshold is dependent on a cell configuration, a type of coordinated communications, and the like, it is difficult to determine an optimal threshold uniquely. Moreover, a too large threshold results in the excessive number of coordination candidate cells, and the processing amount at a base station, and the amount of signals communicated between the base station and user equipment increase as described above. On the other hand, a too small threshold results in the excessively small number of coordination candidate cells, and the gain of the coordinated communication decreases. As for the types of downlink coordinated communications, for example, there are a coherent JT, a non-coherent JT, a dynamic point selection (DPS), muting, and the like.
Because it is difficult to determine an optimal threshold uniquely in a selection method of selecting a coordination candidate cell based on a difference in RSRP and a threshold, a selected coordination candidate cell is not necessarily the optimal one.